1. Field of the Invention
This invention relates to a gain control circuit of the current mirror circuit type suitable for use in a semiconductor integrated circuit.
2. Description of the Prior Art
FIG. 4 shows one example of conventional gain control circuits of the current mirror circuit type which include a current mirror circuit comprising a transistor Q1 and a transistor Q2 in diode connection, an input signal source 9 and a variable resistor R. This gain control circuit further includes an output terminal designated by reference numeral 2. When the value of the variable resistor R connected to the emitter of the transistor Q2 is changed, the voltage V across the resistor is changed, so that the base potential of transistor Q1 varies. This allows the output current I.sub.2 to be changed to thereby control the current gain.
In such a gain control circuit, assuming that the signal current is I.sub.1, the output current is I.sub.2, the voltage across the base and emitter of the transistor Q1 is V.sub.BE1, the forward voltage across the transistor Q2 in diode connection is V.sub.BE2, and the voltage across resistor R is V, the relationships expressed below hold: EQU I.sub.1 =I.sub.S .multidot.A.sub.1 ( 1), EQU I.sub.2 =I.sub.S .multidot.A.sub.2 ( 2),
and EQU V.sub.BE1 =V.sub.BE2 +V (3).
The factor A(A1, A2) in the above equations is defined as follows: EQU A.sub.1 =exp V.sub.BE1 /V.sub.T and EQU A.sub.2 =exp V.sub.BE2 /V.sub.T
where V.sub.T is a thermal voltage and I.sub.s is a backward saturation current. The relationship between the signal current I.sub.1 and the output current I.sub.2 is derived from the above equations (1), (2) and (3) as follows: EQU I.sub.2 =I.sub.1 .multidot.A (4).
The factor A(A.sub.1, A.sub.2) in the above equation (4) can be also expressed as: EQU A=exp V.sub.BE /V.sub.T
since the voltage across the corresponding bases and emitters of transistors formed on the same semiconductor substrate are regarded as being substantially equal to one another. Therefore, the output current I.sub.2 is set to a value A times larger than the input current I.sub.1, as shown in the above equation (4). Namely, by changing the value of the variable resistor R to vary the voltage thereacross, the gain of the amplifier circuit of the current mirror circuit type is set to any desired value.
However, since the variable resistor R is necessary to be connected to the emitter of the transistor Q2 externally in the circuit shown in FIG. 4, an external leading terminal is required. Accordingly, the circuit of FIG. 4, as it is, is not suitable for semiconductor circuit integration.
Ordinarily, there is difficulty in operating the current amplifier type gain control circuit of FIG. 4 by using a low voltage source less than 1 volt. The current mirror circuit constituting the gain control circuit has also the drawback that non-matching of diode characteristics resulting from the combination of transistors of different conductivity types will cause an offset current.
Further drawback with the current mirror circuit type gain control circuit is that distortion of the output may occur by the influence of the input signal.